Isocyanates are produced in large volumes and serve primarily as starting materials for producing polyurethanes. They are mostly produced by reaction of the corresponding amines with phosgene. A distinction is made here between aromatic isocyanates, where the NCO groups are bonded directly to an aromatic ring, and aliphatic or cycloaliphatic isocyanates, where the NCO groups are bonded to an aliphatic, sp3-hybridized C atom.
XDI is an aliphatic diisocyanate with benzylically bonded NCO groups of very specific reactivity.
On account of the high reactivity thereof by comparison with aliphatic polyisocyanates, and the more favorable toxicological properties by comparison with polyisocyanates having aromatically bonded NCO groups, XDI and/or XDI polyisocyanates is/are used advantageously for the adhesive bonding of food packaging. On account of its high refractive index in combination with high light stability, the 1,3-isomer in particular is used for producing optical lenses and spectacle lenses.
The production of XDI in the liquid phase is known practice. EP 0 384 463 B1 claims a process for producing XDI that is characterized by the preparation of XDA hydrochloride in a first reaction stage, at or below 30° C., in an ester as solvent, with the amine hydrochloride being phosgenated in a second reaction step at 120-170° C. The objective of this specific reaction regime is to keep the level of the chloromethyl benzylisocyanate by-product formed at a low level. U.S. Pat. No. 5,523,467 claims a process for producing aliphatic polyisocyanates that is characterized first by reaction of aliphatic polyamines, their hydrochlorides or carbonates with phosgene in an inert solvent, the reaction then being continued with additional introduction of an inert gas. Under the conditions of the inert gas introduction, the phosgene excess in the production of the XDI can be kept at a lower level, and the formation of unwanted tar during the reaction can be reduced. A disadvantage of this method is that it is a multistage batch operation which is carried out in high dilution in a relatively high-boiling solvent and requires long reaction times. The laborious XDI production method is complex, energy-intensive, and therefore expensive.
EP 1 908 749 A1 describes the phosgenation of amine hydrochlorides, particularly of 1,3-XDA hydrochloride, where the amine hydrochlorides are produced under superatmospheric pressure, in order to control the particle size in the suspension, in order to lower the viscosity of the slurry, and in order to generate a low level of secondary components, especially of 3-chloromethyl benzylisocyanate, during the phosgenation. The heterogeneous phosgenating reaction of the salts of XDA in high dilution in specific solvents while observing a defined reaction regime in order to avoid the formation of tar or of the unwanted secondary component 3- and/or 4-chloromethyl benzylisocyanate (3-chloroxylylene isocyanate or 3-CI-XI; 4-chloroxylylene isocyanate or 4-CI-XI, also referred to, individually or in a mixture, as CI-XI) is difficult to realize and is accompanied by large losses in yield.
All of these difficulties have to date stood in the way of the more broad use of XDI.
One particularly economic possibility for the production of isocyanates is the reaction of the corresponding amines with phosgene in the gas phase.
EP 0 289 840 B1 describes a method for producing (cyclo)aliphatic diisocyanates by phosgenation of the corresponding, vaporous (cyclo)aliphatic diamines at 200° C. to 600° C. Phosgene is supplied in a stoichiometric excess. The superheated streams of vaporous (cyclo)aliphatic diamine or (cyclo)aliphatic diamine/inert gas mixture, on the one hand, and of phosgene, on the other, are passed continuously into a cylindrical reaction space, where they are mixed with one another and reacted. During the exothermic phosgenating reaction, a turbulent flow is maintained.
EP-A 0 928 785, EP-A 1 319 655, EP-A 1 555 258, EP-A 1 275 639, EP-A 1 275 640, EP-A 1 403 248 and EP-A 1 526 129 describe specific embodiments for this technology, which, however, relate to the reactor per se and the reaction regime, without addressing in more detail the vaporizer technology used in the pretreatment of the reactants.
EP 0 593 334 B1 (U.S. Pat. No. 5,391,683) and EP 0 699 657 B1 (U.S. Pat. No. 5,679,839) claim methods for producing aromatic polyisocyanates. The reaction of xylylenediamine, as well as tolylenediamine and phenylenediamine, with phosgene is mentioned, but the reference in both cases is to diaminoxylene. US 2012/0095255 A claims a method for producing isocyanates wherein an amine and phosgene can be mixed with one another and reacted in the gas phase. One dependent claim (claim 26) also claims the reaction of p-xylylenediamine, within a long series of mono-, di-, and triamines. None of these applications contains any references to the particular difficulties which arise with the vaporization of XDA, or the overcoming of these difficulties.
EP 1 754 698 B1 describes a method for producing isocyanates by phosgenation of amines in the gas phase, wherein the amines are heated in liquid form, vaporized and/or superheated in gas form using one or more heat exchangers having a volume-specific heat exchanger area for the amine side of at least 1000 m2/m3 and which, for guiding the flow of the amines, have channels which possess a hydraulic diameter of 1000 to 10 000 μm. This method as well is not suitable for phosgenating XDA, since the small dimensions of the channels of these heat exchangers mean that they very quickly become blocked by the xylylenepolyamines which form, these polyamines being of high viscosity and becoming solid on further temperature exposure.
WO 2013 079517 A1 describes a specific method for producing isocyanates by gas-phase phosgenation. In this particular embodiment, a method for producing isocyanates by phosgenation of the corresponding amines in a fluidized-bed reactor is claimed, the method being characterized in that a gas stream containing the phosgene is utilized as fluidizing gas to maintain an inert solid in suspension, and in that a liquid stream containing the amine is metered into the fluidized bed, with the amine undergoing partial or complete vaporization and reacting with the phosgene to give a reaction gas mixture comprising the corresponding isocyanate, which is taken off from the fluidized-bed reactor. The utilization of the heat of reaction for vaporization and superheating of the amines in the fluidized bed is seen as an advantage. What that specification conveys is that the only amines suitable for gas-phase phosgenation are thermally stable amines which undergo only a relatively low degree of decomposition at the high vaporization temperatures required—at most 2 mol %, more preferably at most 1 mol %, or very preferably at most 0.5 mol %. These thermally stable amines include aliphatic diamines and aromatic diamines, which are listed by way of example in the specification. Benzylic amines do not belong to this group, as described below on the basis of our own results.
As our own experiments, however, have shown, the gas-phase methods according to the prior art described cannot be used readily for the phosgenation of XDA, since XDAs form nonvolatile xylylene-polyamines even at temperatures well below their boiling temperature, giving off ammonia as they do so. As a result, after just a short time, there is an increase in viscosity, there is fouling and clogging in the amine vaporizer, and there is fouling caused by deposition of ammonium chloride in the reactor and in the downstream offgas system.
In summary it may be stated that to date no method from the prior art is suitable for the production of XDI in the gas phase. Consequently there continued to be a great need for a simple and inexpensive method for producing XDI in the gas phase, avoiding the disadvantages described for the methods of the prior art.